1. Field of the Invention
The present invention relates to a semiconductor device formed using an oxide semiconductor and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device refers to all the devices that operate by utilizing semiconductor characteristics. A transistor in this specification is a semiconductor device, and a display device, a semiconductor circuit, and an electronic device including the transistor are all semiconductor devices.
2. Description of the Related Art
Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.
Attention has been directed to a technique in which, instead of the above silicon semiconductors, metal oxides exhibiting semiconductor characteristics are used for transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, a technique is disclosed in which a transistor is formed using zinc oxide or an In—Ga—Zn-based metal oxide as an oxide semiconductor and such a transistor is used as a switching element or the like in a pixel of a display device (see Patent Documents 1 and 2).
It is pointed out that hydrogen contained in an oxide semiconductor serves as a carrier supply source. Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor at the time of depositing the oxide semiconductor. Further, shift of the threshold voltage of a transistor formed using an oxide semiconductor is suppressed by reducing the amount of hydrogen contained in not only the oxide semiconductor but also a gate insulating film in contact with the oxide semiconductor (see Patent Document 3).